The present invention relates to magnetic storage devices and, more particularly, to computer disk drives. More specifically, the present invention relates to compensating for amplitude and BER (bit error rate) loss due to media thermal decay.
Computer disk drives store digital information on magnetic disks which are coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. Typically, the digital information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk.
More specifically, during operation of a conventional disk drive, a magnetic transducer is placed above a desired track of the disk while the disk is spinning. Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track which constitute the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly-timed digital signal that can be recognized by a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Typically, transducers include separate elements for reading and writing. Such transducers are known as xe2x80x9cdual element headsxe2x80x9d and usually include a magneto-resistive (MR) read element or giant magneto-resistive (GMR) read element for performing the read function.
Dual element heads are advantageous because each element of the transducer can be optimized to perform its particular function. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and, thus, can read much fainter signals from the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface with no loss of read performance.
MR read elements generally include a strip of magneto-resistive material that is held between two magnetic shields. The resistance of the magneto-resistive material varies almost linearly with applied magnetic field. During a read operation, the MR strip is held near a desired track, with the varying magnetic field caused by the magnetic transitions on the track. A constant DC current is passed through the strip resulting in a variable voltage across the strip. By Ohm""s law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host. GMR read elements operate in a similar manner.
FIGS. 1(a)-1(e) are simplified diagrammatic representations which illustrate how data is written as transitions on a disk surface and how the transitions are read from the disk surface as data. As background, a transition is where the magnetization in the disk media changes. In general, there are two types of transitions possible; that is, where south poles face south poles and where north poles face north poles.
FIGS. 1(a)-(c) illustrate the write process in simplified form. Specifically, FIG. 1(a) illustrates a data sequence in the form of xe2x80x9conesxe2x80x9d and xe2x80x9czeros,xe2x80x9d which is to be stored on the disk media. FIG. 1(b) illustrates the write current in the write coil for one method of storing the data sequence. In such method, the current through the write coil is reversed at each xe2x80x9conexe2x80x9d and remains the same at each xe2x80x9czeroxe2x80x9d (see FIGS. 1(a) and 1(b)). Consequently, as the disk media is rotated under the write head, the disk media is magnetized as shown in FIG. 1(c). It should be noted that magnetic transitions occur at each xe2x80x9conexe2x80x9d and not at each xe2x80x9czero.xe2x80x9d It should also be noted that FIG. 1(c) represents the magnetization of the media for a portion of a track, which is shown in a linear rather than arcuate shape, as will be understood by those skilled in the art.
FIGS. 1(d) and 1(e) illustrate the read process in simplified form. As mentioned above, as the disk media is rotated under the read head, a constant DC current is passed through the MR strip in the read head. The magnetic transitions stored in the disk media cause the magnetic field applied to the MR strip in the read head to vary, as shown in FIG. 1(d). Since the resistance of the magneto-resistive material varies almost linearly with applied magnetic field, the varying magnetic field caused by the magnetic transitions on the disk media results in a variable voltage across the strip. By Ohm""s law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track, as shown in FIG. 1(e). The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
The amount of information capable of being stored on a disk surface is determined, in part, by the minimum size of individual transitions. As is known to those skilled in the art, the minimum size of individual transitions is based (among other things) upon the grain size of the magnetic material forming the magnetic layer of the disk surface. In order to increase the amount of information capable of being stored on the disk surface, disk manufacturers have been continuously reducing the grain size of the magnetic material and, hence, have reduced the minimum size of individual transitions. For the magnetic layer of the disk, the remnant magnetization-thickness product has also been reduced to achieve higher linear densities and enhanced writer performance. Most of this reduction has been achieved by reducing the thickness of the magnetic layer of the disk, and hence, the grain thickness, which reduces the grain size.
Traditionally, about 500 to 1000 grains of magnetic material were required to store a bit of information. However, at present, a transition may be stored in about 100 grains of magnetic material. It is expected that the number of grains of magnetic material required to store a bit of information will continue to decrease over time. To reduce transition noise and increase the number of grains in a transition, both the diameter of the grains and the separation between the grains have been decreased. In fact, the diameter of the grains has decreased from approximately 15 nm down to approximately 9-10 nm. This has driven disk vendors to produce disks with smaller grain volumes.
As will be understood by those skilled in the art, each grain has a certain magnetic anisotropy energy associated with it. More specifically, the anisotropy energy of a grain is a fixed amount of energy required to xe2x80x9choldxe2x80x9d a stored direction of magnetization in the magnetic material, and is equal to the anisotropy energy density, Ku, times the volume of the grain, V. A thermal instability ratio is defined as the anisotropy energy divided by the thermal energy, KT, and is given by the formula KuV/KT, which should be greater than 50 for adequate thermal stability.
As grain sizes have been reduced, the anisotropy energy associated with each grain has been reduced. In fact, the anisotropy energy of each grain has been reduced such that it is comparable to the ambient thermal energy in the disk drive. Consequently, the thermal energy in the disk drive randomly excites grains in the magnetic material causing changes in the direction of magnetization of the magnetic material over time. Ultimately, if a threshold number of grains change their direction of magnetization, information stored on the disk may be lost. This phenomenon is known as the superparamagnetic effect (or thermal decay). Furthermore, the superparamagnetic limit is defined as the threshold number of grains in which magnetization changes must occur, due to thermal decay, to cause a loss of information.
In other words, the superparamagnetic effect is a thermal relaxation of information stored on the disk surface. Because the superparamagnetic effect may occur at room temperature, over time, information stored on the disk surface will begin to decay. Once the stored information decays beyond a threshold level, it will be unable to be properly read by the read head and the information will be lost.
More specifically, the superparamagnetic effect manifests itself by a loss in amplitude in the readback signal over time. Accordingly, this causes the bit error rate (BER) to increase. As is well known, the BER is the ultimate measure of drive performance in a disk drive.
In general, a certain number of bit errors may be corrected by a disk drive""s error correction code (ECC). However, as is well-known, ECC information adds to the overall overhead of a disk drive, which limits the amount of information that can be stored on a disk surface. Accordingly, disk drives exhibiting high bit error rates that require correction by a large amount of ECC overhead are generally disfavored.
Without a doubt, the superparamagnetic effect poses significant problems in the disk drive industry, where information must be reliably stored and reliably recovered. In order to overcome the problems associated with the superparamagnetic effect, at least one disk-side solution has been proposed.
For example, disk manufacturers have tried to make disks having grains of magnetic material with higher anisotropy energies. In order to increase anisotropy energies in the grains, larger grain sizes are generally used. Using larger grain sizes, however, causes signal-to-noise problems. Specifically, larger grains produce more transition noise, which limits the SNR of the system and ultimately the BER. If the anisotropy energy is increased by increasing the anisotropy constant, Ku, which increases the coercivity, Hc, it becomes difficult to write transitions on the disk.
In addition to the above, there are additional reasons why disk-side solutions may be insufficient. Specifically, properties of the disks may vary from disk-to-disk, not only between different disk manufacturers, but also between the same disk manufacturer. For example, the grain size and anisotropy energies of the magnetic material may vary between disks of the same manufacturer.
Furthermore, while the superparamagnetic effect may occur at room temperature (i.e., nominally 25 degrees Centigrade), disk drives operating at higher temperatures are more susceptible to the problems associated with the superparamagnetic effect. Due to the fact that most disk drives are located within a PC chassis and such chassis are filled with electronic circuitry, many disk drives are operated at temperatures of about 60 degrees Centigrade. Others are operated at even higher temperatures, depending (in part) upon the characteristics of the PC chassis. Thus, while a group of disks manufactured by one or more manufacturers may not be seriously affected by the problems associated with the superparamagnetic effect in a particular disk drive, the same group of disks may suffer from problems associated with the superparamagnetic effect in other disk drives.
Accordingly, it would be advantageous to provide a drive-based (also known as drive-level) solution to the problems associated with the superparamagnetic effect, so that accommodations may be made for the varying properties associated with individual disks in a disk drive and so that accommodations may be made for variations in operating temperatures of disk drives. Furthermore, it would be advantageous to provide a drive-level solution to the problems associated with the superparamagnetic effect which is adaptive, so that corrective action may be taken based upon the extent of thermal decay that has occurred.
The present invention is designed to overcome the aforementioned problems and meet the aforementioned, and other, needs.
In one embodiment, a method is provided for supplying adaptive drive-level compensation for amplitude and BER loss due to media thermal decay. Specifically, a disk surface is provided, wherein the disk surface has information stored thereon. The information is written in the form of magnetic transitions, which are subject to thermal decay. A head is used to read the information stored on the disk surface and is biased by a bias current (or a bias voltage).
In order to compensate for thermal decay of the information stored on the disk surface, high frequency and BER reference sectors are written near the inner diameter of the disk surface. During a drive initialization procedure, initial values read from both the high frequency reference sector and the BER reference sector are stored in memory.
Upon the expiration of a specified amount of time, the high frequency reference sector and the BER reference sector are read, and their values are compared to their initial values to determine the extent of thermal decay of the disk surface. If either of the values fall outside of a predetermined specification relative to the initial values, an opti routine is performed.
In the opti routine, the bias current (or bias voltage) is incrementally raised, in an effort to compensate for the thermal decay and to bring the values within the predetermined specification. However, the bias current (or bias voltage) may only be raised up to a predetermined bias current limit (or bias voltage limit).
Once the bias current limit (or bias voltage limit) is reached, the information stored on the disk surface is re-written. Furthermore, the bias current (or bias voltage) preferably is then returned to its initial value. Since thermal decay is a function of time, rewriting the information on the disk surface causes the information to be refreshed. Thus, the amplitude of the high frequency reference sector and the BER of the BER reference sector will be brought to acceptable levels.
In order to ensure that the drive is not powered down during the re-write process, a message is preferably generated to let a user know that the disk surface is being rewritten and that power should not be shut off. Optionally, the data can be redundantly moved into the computer""s RAM or disk drive""s RAM, rewritten, verified and then deleted from RAM (or simply overwritten).
It should be understood that this summary section is only intended to provide an overview of the invention. Furthermore, this summary section does not necessarily disclose all features and embodiments of the invention. Instead, further details are provided in the detailed description section and the drawings. Thus, other objects, embodiments, features, aspects and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.